Many known explosives require significant shock, heat, force or other stimuli to detonate, generally referred to as a secondary explosive. As such, an explosive train is often used to efficiently detonate these explosives, where the explosive train often includes a detonator and an intermediary. To provide ease of use, detonators generally are constructed using easily detonated primary explosives.
Given the ease of ignition of a detonator, precautions are taken to prevent accidental initiation of the detonator or to interrupt the explosive train extending to the explosive.
A first known approach is to physically isolate the detonator from the rest of the explosive train until just before the desired detonation. This requires an operator to physically connect the detonator to the rest of the explosive train at the final location of usage. While advantageous in that the explosive train is not complete prior to connection, the initiation device must be connected before detonation is needed and, in perforating a wellbore, before the explosive is positioned.
Another approach is the use of a deflagration to detonation device, exploding bridgewire or exploding foil initiator to directly detonate an explosive train constructed solely of secondary explosives. While effective, these systems are limited by the technology available, reliability and/or the high cost and complexity of the electrical systems.
An alternative approach includes interrupting the explosive train so that, even if the primary explosive detonator is initiated, at least a portion of the explosive train is not “in line” with the rest, so that the explosive at the end of the explosive train is left undetonated. These systems generally can be classified as either blocking or misaligned. In a blocking system, a barrier or other blockage is positioned so as to interrupt the explosive train. In practice, while the barrier may be exposed to the detonator or other portion of the explosive train, the barrier prevents the explosive train from continuing therepast.
In a misaligned system, at least one portion of the explosive train is shifted so as to not be aligned with the rest of the explosive train. With the system misaligned, the progress of the explosive train is limited by the misaligned location, thereby ending the explosive train extending between the detonator and the explosive. However, with the misaligned portion shifted back into aligned with the remainder, the explosive train can be initiated and maintained to detonate the explosive.
One method of accomplishing the interruption of the explosive train, whether misaligned or blocked, is for an operator to physically remove the barrier or realign the explosive train prior to use. This allows for a safe system up to the point of being physically manipulated. However, once realigned or unblocked, the explosive train is intact. As such, physically interacting with the arming device requires access to the arming device and may result in further handling the armed device prior to actual use.
An alternative method is utilized in ballistic applications in which the interrupted system automatically shifts to an uninterrupted state (i.e., unblocked or aligned) upon the presence of specific external forces or environmental conditions. For a given application, specific environmental or external factors associated with a desired arming condition are determined. For example, a specific impact force applied to the arming device, velocity of the arming device or angular rotation of the arming device can be utilized. Additionally, environmental factors, such as pressure or temperature, can be utilized to transition an arming device to an armed state. However, care must be taken in the selection of the external forces and environmental conditions utilized to arm the arming device as once the external force or environmental condition is encountered the arming device will be armed whether intended or not.